Enabling the I/O MMU Bypass Mode on a PCI Bus

If you have an Infiniband Host Channel Adapter (HCA) card, you might need to turn the I/O memory management unit (MMU) bypass mode on. By default, Logical Domains software controls PCI-E transactions so that a given I/O device or PCI-E option can only access the physical memory assigned within the I/O domain. Any attempt to access memory of another guest domain is prevented by the I/O MMU. This provides a higher level of security between the I/O domain and all other domains. However, in the rare case where a PCI-E or PCI-X option card does not load or operate with the I/O MMU bypass mode off, this option allows you to turn the I/O MMU bypass mode on. However, if you turn the bypass mode on, there no longer is a hardware-enforced protection of memory accesses from the I/O domain.

The bypass=on option turns on the I/O MMU bypass mode. This bypass mode should be enabled only if the respective I/O domain and I/O devices within that I/O domain are trusted by all guest domains. This example turns on the bypass mode.